<!DOCTYPE html>
<html>
    <head>
        <style>
            #aaa{
               border:1px solid green; 
            }
            .a{
                width:200px;
                height:200px;
                border:1px solid yellow;

                float:left;
            }
        </style>
    </head>
    <body>
        <div id='aaa'>
            <div class='a'>1</div>
            <div class='a'>2</div>
            <div style="clear:both"></div>
        </div>
    </body>
</html>
